1. Field of the Invention
The present invention relates to selectively called receivers, so-called "pocket bells", which generate a predetermined bell sound in response to a call from a calling end and, more particularly, to improvements to the structure of such receivers which respond to such call with high reliability under whatever conditions.
2. Description of the Prior Art
In order to selectively call such a selectively called receiver to cause same to perform a predetermined ringing operation, a signal sent from a calling transmission end to these selectively called receivers generally has a frame structure shown in FIG. 1.
The transmitted signal includes a one-frame signal transmitted repeatedly at predetermined periods, the signal being time divided into a plurality of "groups" (the number of groups in this example=m), as shown in FIG. 1(a), each group being, in turn, time divided into a "synchronizing word" and a plurality of "call words" (the number of words in this example=n) in the format shown in FIG. 1(b). As shown in FIG. 1(c), the "synchronizing word" includes a "bit synchronizing signal" which includes a predetermined number of bits of alternate logical values "0" and "1" for each bit of the frame signal, and a "group synchronizing signal" which accommodates the appropriate group number of the frame signal as data (symbol).
In the frame signal of FIG. 1, the "call word" is a portion which has an "individual number symbol" corresponding to a respective selectively called receiver, and the "group" is a unit allocated to the frame signal as an element which generally calls any number of selectively called receivers. An individual receiver which a subscriber has necessarily belongs to a respective one of a first to a m.sup.th "groups", and has a "call word" allocated thereto corresponding to the "individual number" in the appropriate "group".
Each receiver usually receives intermittently only a signal related to the "group" portion in one frame signal to which that receiver belongs, and checks whether the "call word" indicative of the "individual number" allocated beforehand to that receiver is present in the received signal. When the receiver detects that the "call word" indicative of the "individual number" is present, it performs a ringing operation to inform the appropriate subscriber (the user or owner of the receiver) that there is an incoming call for the subscriber.
The respective "groups" constituting the frame signal appear repeatedly at predetermined periods for each frame. If each receiver can only recognize the contents of the "group synchronizing signal" from the first received signal after the power source is turned on, it can easily realize intermittent reception of only a signal involving the "group" portion to which that receiver belongs in the manner mentioned above after the turning on of the power source in accordance with an appropriate timekeeping operation on the basis of the comparison of the contents of the "group synchronizing signal" with the group to which the receiver belongs.
FIG. 2 illustrates one example of the structure of a receiver conventionally employed generally as a selectively called receiver. As shown in FIG. 2, the receiver includes a power source switch 1, a power source (batteries) 2 which is put under a power supply enable state by turning on the switch 1, a battery saving circuit 3 which mainly controls the power feeding from the power source 2 on the basis of a received signal at the early turning on of the power switch 1 in order to perform an intermittent reception, a symbol memory 4 comprising a non-volatile memory in which a symbol indicative of the number of the "group" to which the receiver belongs and a symbol indicative of the "individual number" of the receiver are beforehand recorded, a receiving antenna 5, a radio circuit 6 which receives and processes the frame signal (see FIG. 1; actually, this frame signal is modulated as required and then transmitted) via the receiving antenna 5, a demodulator 7 which demodulates the received signal properly, a waveform shaping circuit 8 which binarizes the demodulated signal on the basis of a predetermined reference level signal and shapes the result to a pulse signal, a symbol check circuit 9 which compares and checks the logical value structure of the shaped pulse signal with the logical value structure of a symbol indicative of the "individual number" recorded in the code memory 4 and outputs a ringing signal output command BC2 when those logical value structures coincide, a ringing signal forming circuit 10 which forms and outputs a predetermined ringing signal such as the "bell sound" on the basis of application of the ringing signal output command, an amplifier 11 which amplifies the formed ringing signal properly, and a speaker 12 which generates a ringing tone such as a "bell sound" on the basis of the amplified ringing signal.
The operation of the receiver will now be outlined with reference to FIG. 3. Assume that when the frame signal is being transmitted in the format shown in FIG. 3(a), the power source switch 1 is turned on by the subscriber or user with the timing, for example, shown in FIG. 3(b). The battery saving circuit 3 first feeds (supplies a power supply signal SP) simultaneously to all the respective circuits 6-11 to enable the respective circuits to operate. Thus, the frame signal is received via the antenna 5 by the radio circuit 6, demodulated by the demodulator 7, and shaped by the waveform shaping circuit 8 to a pulse signal having the form mentioned above.
The battery saving circuit 3 then extracts the shaped frame signal, searches the "synchronizing word" (see FIG. 1) in the frame signal, compares and checks the "synchronizing word" and a symbol indicative of the "group" number of the receiver recorded in the symbol memory 4 and performs the synchronization of the intermittent reception on the basis of the result of the comparison and check.
The battery saving circuit 3 then supplies the power supply signal SP to the respective circuits 6-11 for a constant time duration at a constant time period on the basis of the form of the resulting synchronization such that only a signal of the "group" portion of the frame signal to which that receiver belongs is received. Generally, in consideration of the start of the operation of the radio circuit 6 and demodulator 7, the timing at which the power supply signal SP is applied is slightly advanced (see FIG. 3(c)).
Each time the intermittent reception is carried out in such manner, the symbol check circuit 9 compares and checks a symbol indicative of the "individual number" in the manner mentioned above. When the coincidence is detected, the symbol check circuit 9 determines that there is an incoming call to that receiver and outputs a ringing tone signal output command BC2 to the ringing tone signal forming circuit 10 which then forms and outputs the ringing signal. This signal is properly amplified by the amplifier 11 and sounded as the ringing tone via the speaker 12.
The waveform shaping circuit 8 which supplies a reproduced version of the frame signal to the battery saving circuit 3 and symbol check circuit 9 determines the binarization of the demodulated output from the demodulator 7 using a predetermined reference level signal, as mentioned above. In order to provide a correct reproduced version of the frame signal, the determination of the binarization is required to be performed with high accuracy, of course.
In order that the reference level signal always has an appropriate value, the waveform shaping circuit 8 uses the "bit synchronizing signal (0, 1, 0, 1, . . . )" of the "synchronizing word" to hold the average value of the demodulated output of the "bit synchronizing signal (0, 1, 0, 1, . . . )" as the reference level signal for the shaping circuit 8 and maintains the held value so as not to fluctuate during other time intervals. As the shaping circuit 8, a circuit such as that illustrated in FIG. 2 is usually used.
As shown in FIG. 2, the shaping circuit 8 basically includes a comparator 81, a capacitor 82, a charge control switch 83, and a resistor 84. The switch 83 is turned on (closed) only during the reception and demodulation of the "bit synchronizing signal" to thereby rapidly charge and discharge the capacitor 82. During the time interval excluding the reception and demodulation of the "bit synchronizing signal", the switch 83 is turned off (opened) to increase the time constant related to the capacitor 82 to thereby suppress to a minimum degree fluctuations of the voltage held by the capacitor 82 which will be the reference level signal. The rapid charge and discharge of the capacitor 82 based on the turning on of the switch 83 is performed only for a time duration in which the hold voltage in the capacitor 82 can be the reference level signal even when the power source switch 1 is turned on. This is so because the contents of the received frame signal are intended to be reproduced accurately even after the power switch is turned on. The turning on/off of the switch 83 are controlled systematically by the battery saving circuit 3. A signal CC shown in FIG. 2 is a charge control signal which controls the turning on and off of the switch 83. The manner in which the charge control signal CC is generated is illustrated in FIG. 3(d). The switch 83 is turned on when the control signal CC is on (when the logical level is high) and is controlled so as to be off during other time intervals.
In the receiver, generally, when the voltage fed by the power source (battery) 2 is maintained higher than a predetermined value which is capable of driving the respective circuits 6-11 when the power source switch 1 is turned on, the receiver informs the subscriber or user, using a ringing tone such as "bell sound", that the receiver is usable. At this time, the battery saving circuit 3 checks the output voltage from the power source 2 on the basis of the turning on of the power source switch 1. If the output voltage is higher than the predetermined value, a ringing tone signal output command BC1 is applied to the ringing tone signal forming circuit 10 to expedite the formation and outputting of the ringing tone signal. FIG. 3(e) shows the manner in which the ringing tone signal output command BC1 is generated.
As just described above, the conventional selectively called receiver simultaneously sets a reference level for the waveform shaping circuit 8 and gives a command for outputting a ringing signal to the ringing tone signal forming circuit 10 when the power source switch is turned on (see FIGS. 3(d) and (e)). Therefore, when the reference level is set, namely, when the capacitor 82 is rapidly charged and discharged, a big current flows through the ringing signal forming circuit 10, amplifier 11 and speaker 12 due to the ringing operation, so that it is difficult to set and maintain the reference level at the appropriate value mentioned above. So long as the reference level is not maintained at the appropriate value, the frame signal cannot be reproduced accurately and, furthermore, a call through the frames signal (or to report to the subscriber), of course, cannot be responded properly.